staging: most: net: fix buffer overflow

staging: most: net: fix buffer overflow

If the length of the socket buffer is 0xFFFFFFFF (max size for an
unsigned int), then payload_len becomes 0xFFFFFFF1 after subtracting 14
(ETH_HLEN).  Then, mdp_len is set to payload_len + 16 (MDP_HDR_LEN)
which overflows and results in a value of 2.  These values for
payload_len and mdp_len will pass current buffer size checks.

This patch checks if derived from skb->len sum may overflow.

The check is based on the following idea:

For any `unsigned V1, V2` and derived `unsigned SUM = V1 + V2`,
`V1 + V2` overflows iif `SUM < V1`.

Reported-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Andrey Shvetsov <andrey.shvetsov@k2l.de>
Cc: stable <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/20200116172238.6046-1-andrey.shvetsov@microchip.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>